1. Field of the Invention
The present invention relates to an optical pickup device used in a recording and reproducing apparatus for an optical recording medium, such as an optical disk, etc., and more particularly to an optical pickup device that is capable of controlling the optimum focusing position of light flux focused on a target recording surface of an optical recording medium, such as an optical disk, etc., having a plurality of recording layers stacked using an astigmatic method.
2. Description of the Related Art
In recent years, an optical disk has been widely used as a means to record and reproduce various kinds of data, such as video data, audio data, computer data, etc. A high-density recording type disk called a Blu-ray™ Disk (hereinafter, referred to as a BD) has been put to practical use. This optical disk standard encompasses a multi-layer optical disk in which a plurality of recording layers are laminated. In a multi-layer optical disk constructed in a structure in which a plurality of recording layers are alternately stacked while spacer layers are interposed respectively between the recording layers, it is required to adjust the focus of light (an in-focus position or the optimum focusing position) on the recording surface of a target layer and irradiate the focused light spot to a target recording layer, in order for information to be read from one surface side of the optical disk by an optical pickup device.
For example, as shown in FIG. 1, a dual-layer disk is constructed in a structure in which a first recording layer (hereinafter, referred to L0, formed of a semitransparent film, which is close to a reading side thereof, and a second recording layer (hereinafter, referred to L1), formed of a reflective film, such as a metal or a dielectric, are disposed on a substrate Sub, a light-transmissive spacer layer SP to separate the recording layers from each other by a predetermined thickness is interposed between L0 and L1, and a cover layer CL is disposed to protect L0.
In case of the spacer thickness being large, when the focus is adjusted, for example, on L0, which is a target layer, from the cover layer side of the disk, laser light L2LB focused on L1 widely spreads out, with the result that light reflected from L1 is not bit modulated and thus becomes a very low frequency or direct-current-like signal. For this reason, when a high-frequency component is drawn from the read signal by a high pass filer, it is possible to read only the signal from L0. In case of the spacer thickness being small, on the other hand, even when the focus is adjusted on L0, laser light irradiated to L1 does not widely spread out. As a result, a signal from L1 leaks to some extent. (This leakage is called an interlayer cross talk.) Even in case of laser light L1LB being focused on L1, which is a target layer, the same result is obtained.
In order to adjust the focus on a target recording layer of a multi-layer optical disk, it is necessary to generate a focus error signal and perform servo control (focus draw-in). In order to prevent focus error noise, on the other hand, it is necessary to eliminate an influence, such as the interlayer cross talk, from the focus error signal.
Even in case of the interlayer cross talk being restrained, however, when laser light is focused on L0, which is a target layer, reflected light (signal light) is guided to an photo detector by a condensing lens, and, on the other hand, the reflected light (stray light) component of light having passed through L0 and spread out at L1 is also incident on the photo detector as stray light having a predetermined spread-out.
The stray light except the signal light interferes with the signal light, with the result that noise may occur. Also, the quality of an output signal from the photo detector may be deteriorated, and defects, such as the offset of a servo error signal, may occur.
For the pickup device, the reduction of noise caused by the stray light is much more pursued. In order to prevent the incidence of the stray light on the photo detector and thus avoid the interference between the stray light and the signal light in the photo detector, therefore, there has been proposed a technology for preventing such interference by masking some of returning light such that light from a layer other than a target recording layer is not directly incident on the photo detector (See Patent document 1).
Patent document 1: Japanese Patent Application Publication No. 2005-63595